My DS is nearly 10 and has only just become dry at night. Sometimes bed wetting can be a hormone thing but more often it is just teaching the brain and bladder to make the connection over night that "hey I need to pee". Our son woke up drenched every morning right until we started attending a contienence clinic late last year. It took 2 weeks for him to be night trained using the pad and alarm system. Key factors the child psychologist told us were:
Do not restrict fluids before bed
Encourage bladder stretching during the day- encourage lots of drinks and get them to hold on until they can no longer tolerate it. Our sons bladder capacity went from holding 180mls to over 350mls in 2 weeks.
Do not wake the child overnight to go to the toilet, they must learn to wake themselves
At 4.5 he is totally normal and I would not be worried at all, I would just keep him in night pants until he starts waking up dry. Dry nights are Huggies brand night nappies that are designed for older children and are much more absorbent than a regular nappy. Good luck